style(simulation): cargo fmt — assert message wrap in drainage tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/drainage.rs

@@ -1035,6 +1035,9 @@ mod tests {
         let high = classify_river_cell(river_max_accumulation, river_max_accumulation);
         assert_eq!(low, 0, "just above threshold should be a stream");
         assert_eq!(mid, 1, "mid-range accumulation should be a tributary");
-        assert_eq!(high, 2, "the body's max river-cell accumulation should be trunk");
+        assert_eq!(
+            high, 2,
+            "the body's max river-cell accumulation should be trunk"
+        );
     }
 }